Optical Storage

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Optical Storage

• By Javier Romero

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Definition

• Optical data storage refers to storage systems
  that use light for recording and retrieval of
  information. 
• Photographs could be considered analog optical
  storage, but theyre limited in that they cant
  really interface efficiently with a machine.

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Why Optical Storage?

• Light can easily be used for storing and
  gathering digital data with the use of lasers.
• Optical Storage has been considered and is still
  considered to be one of the future replacement
  for magnetic storage. Some advantages of optical
  storage are
• There is a larger distance between the read/write
  device and the actual media, which increases
  reliability and eliminates all form of wear on
  the physical media.
• Portability and removability. You can easily
  take an optical disk and take it anywhere to use
  on any computer with a compatible drive.

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Disadvantages

• The laser in the optical drive can get worn out
  and unstable from repeated use.
• Physical media can have mechanical damage done to
  it from being exposed to the environment.
• Slower random data access. An optical disks RPM
  cannot match a magnetic drives RPM without
  making it vibrate since its usually placed
  loosely in the drive as opposed to a stable fixed
  hard disk in the hard drive. This is what causes
  longer seek times on an optical disk.

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How Data is Stored

• Pre-grooves are physically formed on the plastic.
  The bottom of these grooves are where the data
  is stored, and these grooves can also be
  separated further into lands.
• The laser in the drive travels along these
  grooves. The groove depth is based on the
  wavelength of the laser, and its usually 1/8th
  of the laser beam wavelength. Spots are stored
  along these grooves and represent bits in the
  data
• The laser reads information based off of the
  reflected light from the actual disk spots. The
  device detects the variations in the reflected
  light, which is how the data is read.

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How Data is Stored

• Another way to define tracks and provide servo
  information for the drive's electronics is to use
  a so-called sampled servo, where the tracks are
  defined by occasional marks placed on the
  substrate at regular intervals. The marks define
  the outer limits of the track and help to
  position the laser spot on the track.
• In optical storage its best to have the smallest
  laser spot beam possible, because this allows
  smaller spots to be stored on the disk, which
  gives more area to work with and store more
  possible spots.

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The Laser

• The laser beam from the laser diode passes
  through the diffraction grating to produce two
  secondary beams needed to maintain correct
  tracking of the disk.
• Then, the beam passes polarizing prism (beam
  splitter) and only the vertical polarized light
  passes. The light beam is then converged into a
  parallel beam (by the collimator) and passes
  through the 1/4-wave plate where the beam
  polarization plane is rotated by 45 degrees.
• The beam is then focused onto the disk surface by
  a lens and a servo-controlled mechanism called
  2-axis device. Polarization plane of the
  reflected beam is rotated by another 45 degrees
  turning its initial vertical polarization into a
  horizontal. 
• After a few more reflections, all beams reach six
  photo detectors 4 main spot detectors and 2 side
  spot detectors enabling read-out of the pit
  information from the disk.

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The Physical Media

• The CD is 12 cm in diameter, 1.2 mm thick, has a
  center hole 15 mm in diameter, and spins at a
  constant linear velocity (CLV) or constant
  angular velocity (CAV).
• Unlike the hard disk or floppy disk, there is
  only one track on the optical disk and all data
  are stored in a spiral of about 2 billion small
  shallow pits on the surface.
• Mb/mm2.  The total length of the track on a CD is
  almost 3 miles (4.5 km).

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The Spots

• The compact disk consists of
• The label
• Protective Layer
• Reflective Layer
• Substrate Layer
• The change in reflected light intensity occurs
  every time the laser spot moves from the pit onto
  the land and vice versa. The high-frequency
  modulated signal produced by these changes in
  light intensity  represents the data stored on
  the CD.

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Reliability

• The reason for the high reliability of the CD  is
  good protection of the data from damage both
  inside and outside CD drive.  Outside, the data
  layer is protected by tough 1.2 mm thick layer of
  polycarbonate on one side and 10-20 microns of a
  protective lacquer layer on the other side.
• Small scratches on the surface of CD do not
  directly erase the data, but just create
  additional areas of light scattering. This can
  confuse the drive's electronic, which is also
  much less sensitive to radial scratches than to
  the circumferential ones. 
• Gentle polishing of the scratch can (in many
  cases) make the CD readable again. But, this is
  rarely necessary thanks to the large size of the
  laser spot on the surface of the PC layer - about
  1 mm. 
• This large spot diameter "integrates" the signal
  over the large area making the system much less
  sensitive to dirt and scratches on the disk
  surface.

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From Pits to Bits

• In the compact disk, every transition from pit to
  land and back is interpreted as 1. No transition
  means 0, and the length of each land segment
  represents the number of 0s in the data stream.

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Data Transfer Rates

• The first CD drives played back 75 blocks per
  second, which translated into the data transfer
  rate 1X equal to about 0.15 MB/s. This speed is
  the base to which newer CD-ROM drive speeds today
  are compared to.
• The slower CD-ROM drives, lower than 12x, were
  created to use constant linear velocity. This
  meant that the drive motor had to adjust the
  speed based on how far the read head was from the
  edge of the disk and it allowed a constant data
  transfer rate.
• Eventually they decided to switch to a constant
  angular velocity based drive, which meant the
  disk would spin at the same rate regardless of
  where the read head was. This meant that data
  that was closer to the edge of the disk would be
  read slower than data closer to the center.

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DVD

• DVD used to mean Digital Video Disc, but now it
  means Digital Versatile Disc.
• DVD can be used in the same ways CD can
  Read-only storage, video, audio, and writing and
  reading programs.
• The physical properties of a DVD are identical to
  a CD but it is able to store much more. (0.7 GB
  to 4.7 GB)
• Dual-Layer DVDs (DVD-9) can be used twice as
  much data as single layered DVDs.

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Dual-Layer

• DVD-9 utilizes two layers to store the
  information and two laser beams to retrieve the
  data.
• The first layer is semi-reflective, which allows
  the second beam to reach the second layer, which
  is fully reflective.
• This disk design allows almost twice as much data
  to be stored as DVD-5.  Labels are printed on the
  other side of the disk conventionally.

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Blu-Ray

• It has the same physical properties of CDs and
  DVDs.
• Blu-Ray drives use a blue laser (violet) that has
  a smaller wavelength (405 nm) than the red lasers
  in CD and DVD players.
• By being able to focus on a smaller spot, the
  spot size was reduced and this allows much more
  data to be stored on Blu-Ray discs (50 GB).

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Sources

• Mempile - Optical Storage
• USByte - Optical Data Storage Systems
• Wikipedia - Blu-Ray